Remove unused variables in qremoteobjectnode.cpp

Found by Axivion static analysis.

Commit e77c98352c584bac7b752dde21c93ff09f78b00d started deserializing
the data directly into a ClassSlot instance, but didn't remove the
now unused local variables. Do it now.

The original commit was merged into 6.9, so picking only down to 6.11.

Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV95
Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV96
Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV97
Task-number: QTBUG-144772
Pick-to: 6.12 6.11
Change-Id: I093605cc0bf83d59aba637a256bfb50156d4397a
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>